Diazene fragmentation

The second transformation is a version of the aforementioned Boulton-Katrizky rearrangement [522], Benzofuroxan was not isolated but appeared as an intermediate on heating 2,6-dinitro-3-azidoaryldiazenobenzene. The reaction starts with nucleophilic attack of the diazene fragment on the furoxan cycle nitrogen atom [653]. 

N=N double bond character is indicated, as also supported by generalized valence bond calculations on the simple H2NN.394 Compound 335 dimerizes to the corresponding tetrazene on warming, concomitant with fragmentation via the diradical 356 (Eq. 105).395 Related five-membered 1,1-diazenes fragment to 1,4-diradicals in the gas phase.396... 

Fragments C-N-N-C and S Diazenes and hydrazines with a sulfur source 591... 

Fragments C-N-N-C and S Diazenes and hydrazines with a sulfur source The reaction of 2,3-diazabuta-l,3-dienes with sources of active sulfur to prepare 1,3,4-thiadiazoles has been reviewed in CHEC(1984) <1984CHEC(6)545>, CHEC-II(1996) <1996CHEC-II(4)379> and Chapter 13.12 in the Houben-Weyl Science of Synthesis <2004HOU(13)349>. 

Apparently, the alkylated sulfonylhydrazines fragmented with the spontaneous elimination of o-nitrobenzenesulfinic add to form the diazenes followed by the loss of dinitrogen leading to 15. A benzannulated analog was likewise synthesized [30]. 

A fragmentation reaction which appears to proceed via the generation of 1,4-diradicals is the decomposition of 1,1-tetramethylenediazenes. Unlike the more stable 1,2-diazenes (tetrahy-dropyridazines, see Section 4.2.1.), the 1,1-isomers are not usually isolated or characterized by physical methods but are proposed as intermediates in the thermal decomposition of iV-phenyl-sulfonamidopyrrolidines 1, giving 1,4-diradicals which recombine to yield cyclobutanes 3 and 4. 39 These intermediates are also formed in the photochemical decomposition at low temperature of 1,1-tetramethylenediazenes, prepared in situ from 1-aminopyrrolidines and /er/-butyl hypochlorite.141... 

This is no longer true for cheletropic reactions. When applying the selection rules, one must always consider that the chelating fragment X contributes two electrons. Erroneous conclusions can be made otherwise. Thus, in the cyclopropanation reaction, which should be considered a four-electron cheletropic reaction, only one n bond is broken. In the formation of diazene 32, a six-electron cheletropic reaction, butadiene uses two double bonds and N2 one lone pair and one n bond the two components thus employ a total of eight electrons ... 

If X always contributes two electrons, its chemical nature should be unimportant. This is contradicted by experimental results. Whereas fragmentations of diazenes give good yields and are stereospecific,41 heating of nitrosopyrroline 33 gives, in addition to polymers, only traces of butadiene and N20.42 It can be then be expected that the validity of the selection rules is better for pericyclic than for cheletropic reactions. 

N2 is stabilized by only 11.9 kJ/mol upon coordination to the metal fragment. This electronic stabilization effect is quenched by the zero-point vibrational level differences of 10.8 kJ/mol and becomes unfavorable in view of the reverse entropic effect. trans-Diazene experiences the largest stabilization by —60.3 kJ/mol, while the a-donating ligands are weakly bound. 

We demonstrated how the photoisomerization hypothesis can be supported by accurate quantum chemical calculations (103). The experimental infrared and resonance Raman study of complex 5 led to the first determination of normal modes and force constants of diazene coordinated to a metal fragment. Isotope substitution yielding 15N- and 2H-isotopomers permitted the assignment of diazene normal modes in the experimental spectrum. Moreover, the spectra of these three isotopomers indicated that a laser-induced photoisomerization occurred in the Raman sample. However, a detailed assignment of the split bands was not possible in the experiment. 

Fig. 18. Stereoselection in cycloaddition or fragmentation. Upper two modes of allowed orbital change. Lower decomposition of the diazene obtained from 3-pyrroline (Lemal and McGregor, 1966). The disrotatory path is allowed the conrotatory path would involve a change in orbital symmetry and is forbidden.

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